Sheet of polyphenylene sulfide filaments and process for producing the same

ABSTRACT

This invention relates to a sheet which is formed by randomly dispersing and accumulating polyphenylene sulfide (PPS) filaments and to a process for producing the same. The filaments drawn by a high-velocity air stream are formed directly into a sheet. The sheet is useful as industrial filters, heat insulating materials, electrical insulating materials, etc. because of its outstanding chemical resistance, heat resistance, and electrical insulating properties.

DESCRIPTION Technical Field

The present invention relates to a fibrous sheet which is superior inheat resistance, chemical resistance, flame retardancy, electricalinsulating properties, and strength. More particularly, the inventionrelates to a fibrous sheet of polyphenylene sulfide (referred to as PPShereinafter) filaments and a process for producing the same.

Background Art

PPS is known as a thermoplastic polymer having superior chemicalresistance and heat resistance, as disclosed in U.S. Pat. No. 3,912,695.That PPS is also capable of being melt spun is disclosed in JapanesePatent Publication Nos. 52-12240 (1977) and 52-30609 (1977). Inpractice, however, it is impossible to produce invariably PPS filamentsof uniform quality. In other words, PPS polymer having a viscositysuitable for melt spinning is liable to form particulate gels whichcause quite often breakage of filaments during spinning and drawing.This tendency is pronounced in the case of fine filaments. On the otherhand, if the viscosity is lowered to avoid gelation, the resulting PPSpolymer is so poor in spinnability and so brittle that the filamentsspun from such PPS polymer are easily broken by friction with guides.Furthermore, such PPS filaments produced by some means or other arestiff, liable to static build-up, slippery, lacking the bunchingproperty and even twisting and doubling are difficult to perform, not tomention crimping. It is also difficult to make a uniform sheet from suchPPS filaments by knitting, weaving, or carding. In addition, PPS fibersdo not disperse very well into water because they are extremelyhydrophobic. This makes PPS fibers unsuitable for making sheetstherefrom in the papermaking manner. Such being the case, fibrous sheetsof PPS polymer having high strength and uniform quality have not beenproduced easily.

Disclosure of Invention

The present invention discloses the following.

1. A sheet of PPS filaments which comprises randomly dispersed andaccumulated PPS filaments, each having a fineness of 0.1 to 15 denier.

2. A process for producing a sheet of PPS filaments which comprisesextruding a PPS polymer from a plurality of small holes at a temperature20° to 85° C. higher than the melting point of the PPS polymer, drawingapart the extrudate from the small holes at a rate greater than 1300m/min by a high-velocity air stream, simultaneously causing theresulting filaments to be separated by electrostatic charge, collectingthe separated filaments on a plane, and bonding or interlocking thecollected filaments.

Best Mode for Carrying Out the Invention

According to the process of this invention for producing a sheet of PPSfilaments, a molten PPS polymer is extruded from small holes and thenthe extrudate is conveyed by an air stream, without contact with guidesand rollers. This makes it possible to produce a fibrous sheet withoutfrequent breakage of filaments and troubles of broken filaments beingcaught in rollers. More specifically, the fluid extrudate is introducedto the inside of an annular air jet or the outside of a cylindrical airjet so that the extrudate is drawn apart from the small holes. Inanother way, the molten polymer is extruded from small holes into apressurized compartment and the extrudate is blown out at a high speedtogether with a pressurized fluid ejected from the nozzle opposite tothe small holes. The spinning speed should be at least 1300 m/min,preferably 3000 m/min and over. It is not difficult to realize aspinning speed greater than 5000 m/min, if the above-mentioned method isemployed. According to the process mentioned above, it is possible toproduce fibers having a strength greater than 1.5 g/d and a dry heatshrinkage of 5 to 40% at 140° C. The spinning temperature should be 20°C., preferably 30° to 60° C., higher than the melting point of thepolymer. The small holes have a diameter from 0.1 to 1.0 mm, and thenumber of the small holes is usually greater than ten. The distancebetween the small holes and the air jet is usually 200 to 2000 mm. Tooshort a distance causes filament breakage; and too long a distancedecreases the spinning speed, with a resulting reduced strength offilaments. The adequate distance should be increased or decreased inproportion to the fineness of filaments to be produced.

There is a general trend that the higher the draft ratio, the higher theorientation. The running filaments can be electrostatically charged bybringing a high-voltage electrode into direct contact with the filamentsor by simply bringing it into contact with the guide wall or reflectorof the air jet. Such a method provides a web which is uniform in theweight per unit area.

The web should preferably be treated for static elimination or bondedtemporarily by a hot press so that the web is not disturbed when peeledoff from the collecting plane.

Usually, it is inevitable that the filaments come in slight contact withthe air jet wall surface or the vicinity thereof that generates ahigh-velocity air stream, but it is possible to prevent the filamentsfrom being damaged if the curvature of the contacting surface isincreased or the contacting angle is decreased. This is also true in thecase where the filaments are caused to impinge against a solid face andto rebound from it together with an air stream in order to accomplishseparating.

The process of the present invention provides PPS filaments almost asstrong as the conventional PPS oriented filaments if the spinning speedis increased. In addition, it does not cause necking which is noticed onundrawn filaments. Being made up of filaments, the sheet of thisinvention is superior in strength to nonwoven fabrics made up of shortfibers. The effect of filaments is remarkable when bonding points orinterlocking points are decreased to improve flexibility.

The PPS polymer used in the present invention should have a meltviscosity from 300 to 100,000 poise, preferably 600 to 20,000 poise, ata shear rate of 200 sec⁻¹ at 300° C. One having too low a viscosity canbe increased in viscosity by preliminary curing as disclosed in JapanesePatent Publication No. 52-30609 (1977), at the sacrifice ofspinnability.

According to the present invention, it is also possible to produce mixedfilaments if PPS polymers of different types are extruded from smallholes separated for each type and the extruded filaments are mixed inthe air stream. In such a case, it is also possible to utilize filamentsof one type of PPS polymers as an adhesive for heat-bonding or to causefilaments of one type of PPS polymers to shrink so that the web offilaments is made bulky.

The filaments constituting the web should have a fineness of 0.1 to 15denier, preferably 0.5 to 5 denier, and the web of filaments should havea weight of 10 to 600 g/m², preferably 20 to 300 g/m². Such webs can belaminated with ease. The web of filaments should have an area shrinkageof 5 to 80%, preferably 10 to 60%, more preferably 15 to 40% (measuredaccording to JIS C-2111 providing the method for measuring the areashrinkage of paper), from the view point of subsequent heat treatmentprocess. Any web having an area shrinkage exceeding 80% leads toproducts which are poor in dimensional stability and quality.

The web of filaments of this invention becomes bulky and flexible whensubjected to the slackening heat treatment at 100° to 180° C.,preferably 120° to 160° C., which crimps the constituting filaments. Inaddition, the web may be imparted a variety of characteristics such asbulkiness, strength, flexibility, if the web undergoes interlocking byneedle punching or water jet interlacing prior to the heat treatment.For effective crimping, the needle punching should be performed at adensity of 30 to 300 needles/cm², preferably 50 to 200 needles/cm². Webshaving a weight of 200 g/m² and over are less liable to the damage offilaments and hence maintain high strength. Incidentally, theinterlocking by water jet is preferable for PPS webs because it damagesthe filaments only a little. The method as disclosed in Japanese PatentPublication No. 48-13749 (1973) may be employed. According to thismethod, a web placed on a porous support is subjected to water treatmentcontinuously or intermittently, with the web and support being movedrelatively each other. This method is suitable for thin webs having aweight of 50 to 300 g/m² because the water jet does not penetrate thickwebs easily. Unlike needle punching, the water jet method provides asmooth surface having almost no needle marks.

The term "PPS polymer" as used herein denotes homopolymers or copolymersmade up of p-phenylene sulfide units. They can be obtained bycondensation of p-dichlorobenzene and sodium sulfide. In the case wherea plurality of PPS polymers different in melting points or shrinkage areto be used, selection should be made according to the degree of theircopolymerization. Examples of comonomers include m-dichlorobenzene,1,2,4-trichlorobenzene, and other compounds having a diphenylethergroup, diphenylsulphone group, or naphthalene nucleus. Trichlorobenzeneshould not be copolymerized in an amount more than 1%, because itimpairs the spinnability of the resulting polymer. In the case where aplurality of polymers are used, it is preferable that the content ofcomonomers in the main polymer is less than 10%. Within such a limit,the PPS polymer keeps its fundamental properties regardless of the typesof comonomers used.

The linear PPS polymer of high polymerization degree is especiallysuitable for this invention. Such a polymer is obtained by adding analkali metal salt of carboxylic acid such as lithium acetate at the timeof polymerization.

The degree of crosslinking and branching of a polymer can be defined bythe non-Newtonian constant n represented by the following formula.##EQU1## (wherein r is shear rate, T is shear force, and μ is apparentviscosity.) The value n is obtained by approximation from the plottingof r. It increases in proportion to the degree of crosslinking andbranching. The polymer having 0.9<n<3.0, preferably 0.9<n<2.0, issuitable for this invention.

Such a polymer is superior in spinnability and less liable to gelationduring melt spinning. It is worthy of special mention that the meltspinning speed is remarkably increased--up to 2000 m/min, even up to4000 m/min--when the filaments are taken up by an air stream. Such anextremely high spinning speed is incredible, but is factual. The highspinning speed results in a great increase in strength, Young's modulus,and elongation, and a decrease in shrinkage.

On hot pressing, the sheet of PPS filaments turns to a compact paperlikesheet. If the sheet is allowed to shrink simultaneously with hotpressing, the resulting sheet will be made more compact. Before or afterhot pressing, the sheet may be given a heat resistant binder such aspolyimide, polyamide-imide, aromatic polyamide, polybenzimidazole, andpolyarylenesulfide, in an amount of 5 to 90 wt% based on the filaments,by dipping, coating, spraying, or dusting.

The non-woven fabric of this invention may be made substantiallyinsoluble if treated with an oxidizing agent such as sodiumhypochlorite. Such a product will be useful as industrial filters,firemen's wear, etc. which are exposed to an intense heat.

EXAMPLE 1

Linear high-molecular weight PPS was prepared by reacting 1 mole ofsodium sulfide, 0.14 mole of sodium hydroxide, and 0.90 mole of lithiumacetate in N-methylpyrrolidone under nitrogen at 200° C. withdistillation of water, and further reacting, after adding 1.02 mole ofp-dichlorobenzene, under pressure at 270° C.

The resulting polymer had a melt viscosity of 2900 poise at a shear rateof 200 sec⁻¹ at 300° C. and n=1.25.

This polymer was melted at 340° C. and extruded through a spinnerethaving 20 small holes, each measuring 0.7 mm in diameter, at a rate of0.5 g/min/hole and 1.3 g/min/hole. The extrudate was introduced into anaspirator which was installed 40 cm under the spinneret. The filamentsdischarged from the aspirator were found to have the characteristics asshown in Table 1

The web made up of filaments having the characteristics shown in Table 1was sampled as described in Example 4. The web was found to have aweight of about 350 g/m². The web underwent needle punching with0.028-inch thick needles, each having a triangular cross-section andnine barbs at the tip, at a density of 160 needles/cm². The resultingfelt underwent free shrinkage with hot air at 140° C. blown by a drier,and a piece of bulky felt was obtained. The shrinkage by heat treatmentwas 21% in the longitudinal direction and 25% in the lateral direction.The investigation on the crimp characteristics of some filamentsextracted from the felt revealed helical three-dimensional crimps, withan average of 18 crimps per inch. The felt was found to have thefollowing mechanical properties which are based on the converted weightof 100 g/m².

    ______________________________________                                        Breaking tenacity:                                                                          Longitudinal                                                                             4.3 kg/25 mm width                                                 Lateral    2.7 kg/25 mm width                                   ______________________________________                                    

                                      TABLE 1                                     __________________________________________________________________________         Through-                                                                           Air pres-  Maximum                                                                             Break-                                                                            Break-   Heat shrink-                               put per                                                                            sure of    spinning                                                                            ing te-                                                                           ing ex-                                                                           Young's                                                                            age (160° C.                   Specimen                                                                           hole aspirator                                                                           Fineness                                                                           speed nacity                                                                            tension                                                                           modulus                                                                            × 10 min)                       No.  (g/min)                                                                            (kg/cm.sup.2.G)                                                                     (denier)                                                                           (m/min)                                                                             (g/d)                                                                             (%) (g/d)                                                                              (%)                                   __________________________________________________________________________    1    0.5  0.8   1.44 3120  1.93                                                                              75  19   69.5                                  2    1.3  2.2   1.79 6550  2.16                                                                              40  25   4.0                                   3*   1.3  2.2   1.84 7370  1.71                                                                              34  25   4.1                                   4*   --   --    --   --    2.64                                                                              36  39   --                                    5*   --   --    --   --    2.78                                                                              32  41   --                                    6*   --   --    --   --    2.90                                                                              26  43   --                                    7*   --   --    --   --    1.24                                                                              67  --   --                                    8*   --   --    --   --    2.74                                                                              30  --   --                                    __________________________________________________________________________     3* Melt spinning temperature at 360° C.                                4* Prepared by free heat treatment of specimen No. 2 at 160° C.        5* Prepared by constant length heat treatment of specimen No. 2 at            160° C.                                                                6* Prepared by constant length heat treatment of specimen No. 2 at            200° C.                                                                7* Prepared by free heat treatment of specimen No. 1 at 160° C.        8* Prepared by constant length heat treatment of specimen No. 1 at            160° C.                                                           

    ______________________________________                                        Breaking extension:                                                                          Longitudinal                                                                             147%                                                               Lateral    50%                                                 Tear strength: Longitudinal                                                                             2 kg/tongue method                                                 Lateral    1.5 kg                                              Tenacity at 5% extension:                                                                    Longitudinal                                                                             0.5 kg/25 mm width                                                 Lateral    0.3 kg/25 mm width                                  ______________________________________                                    

These characteristic values, which are comparable to those of theconventional polyester spun bond felt, suggest that the felt in thisexample can be put to practical use. Incidentally, the polyester felttreated under the same conditions gave a breaking strength of 5 to 8 kg,an extension of 80 to 100%, and tear strength of 2 to 4 kg.

On the other hand, an attempt was made to prepare a web by carding fromthe staple of the filaments used in this example. But no web was madebecause of the breakage of fibers. This proved the advantage of makingheat and chemical resistant felt from filaments of PPS polymer.

EXAMPLE 2

Example 1 was repeated except that the density of needle punching forthe resulting web was changed to 10, 50, 200, 300, and 400 per cm². Themechanical properties of the resulting felt are given below as indexvalues, with the values in Example 1 being 100.

    ______________________________________                                        Density of needle punching                                                                     10    50     200   300  400                                  Tenacity at 5% extension                                                                       10    57     130   85   40                                   Breaking tenacity                                                                              20    70     140   90   50                                   Tear strength    10    100     90   70   10                                   ______________________________________                                    

These values indicate that a density of 10 needles/cm² does not providesufficient strength due to insufficient interlocking of filaments, andthat strength rather decreases at a density of 400 needles/cm² becauseof mechanical damage of filaments.

EXAMPLE 3

The web made up of the filaments No. 1 prepared in Example 1 wassubjected to interlocking by water jet as follows: A web having a weightof about 150 g/m² was placed on an 80 mesh metal screen which movesintermittently, and a water jet was applied at a pressure of 70 kg/cm²to the web from a nozzle placed 30 cm above the web, the nozzle having0.13 mm holes arranged in one row at intervals of 3 mm. After dewateringand drying at 110° C., the web underwent heat treatment under a load ofabout 100 g/cm² at 180° C. for 30 minutes. The resulting felt was foundto have an apparent specific gravity of 0.39 g/cc and the followingmechanical properties which are based on the converted weight of 100g/m².

    ______________________________________                                        Breaking tenacity:                                                                           Longitudinal                                                                             8.0 kg/25 mm width                                                 Lateral    6.5 kg/25 mm width                                  Breaking extension:                                                                          Longitudinal                                                                             180%                                                               Lateral    210%                                                Tear strength: Longitudinal                                                                             3.6 kg/tongue method                                               Lateral    2.8 kg                                              Tenacity at 5% extension:                                                                    Longitudinal                                                                             0.8 kg/25 mm width                                                 Lateral    0.7 kg/25 mm width                                  ______________________________________                                    

The resulting feltlike product was found to have a high practical value.

EXAMPLE 4

PPS polymer filaments were prepared as follows from a branched PPSpolymer ("RYTON" made by Phillips Petroleum Co.) having a melting pointof 277° C. and a melt viscosity of 2000 poise at a shear rate of 200sec⁻¹ at 300° C.

This polymer was melted at 320° C. and extruded through a spinnerethaving 20 small holes, each measuring 0.7 mm in diameter, at a rate of0.3 g/min/hole. The extrudate was introduced into an aspirator which wasinstalled 60 cm under the spinneret and was supplied with pressurizedair of 1 kg/cm².G. The PPS filaments were discharged from the aspiratorat a rate of 1700 m/min. The filaments obtained were found to have afineness of 1.6 denier, a breaking tenacity of 1.7 g/denier, a breakingextension of 120%, and a free shrinkage of 45% at 160° C. for 10minutes.

The filament bundle was opened by making it electrostatically charged bya corona discharge apparatus mounted immediately above the aspirator,the apparatus consisting of a needle electrode and a grounding electrodehaving a diameter of 20 mm, with a potential of minus 15000 voltsapplied across the electrodes 8 mm apart. The opened filaments werecollected in the form of a thin layer on a 30 mesh screen placed underthe aspirator.

On reeling the filaments from the web, it was found that the filamentsare distributed as far as 30 cm from the point directly below theaspirator.

A 40 cm wide web having a weight of 55 g/m² was prepared by collectingthe filaments continuously on a moving screen. After eliminating staticcharge, the web was removed from the screen. The non-woven fabric thusprepared was found to have an area shrinkage of 36% when measured for a10×10 cm sample heated in an oven at 160° C.

The non-woven fabric was then passed through calender rolls at 160° C.under a load of 500 kg/m, to be pressed into a 0.2 mm thick compactsmooth paperlike sheet. The sheet was found to be stable enough towithstand a various kinds of wet treatments. As an example, the sheetwas impregnated with a 30% N-methylpyrrolidone solution of polyimide fora pick-up of 25% based on the weight of the filaments. This sheet hadthe following characteristics after curing at 180° C. for 1 hour.

(1) As such

    ______________________________________                                        Breaking tenacity: 3.3 kg/15 mm width                                         Breaking extension:                                                                              10%                                                        Tear strength:     0.7 kg                                                     ______________________________________                                    

(2) After heating at 180° C. for 50 hours

    ______________________________________                                        Breaking tenacity: 3.1 kg/15 mm width                                         Breaking extension:                                                                              7%                                                         Tear strength:     0.7 kg                                                     ______________________________________                                    

In the meantime, when the air pressure for the aspirator was increasedto 1.5 kg/cm², it was impossible to make a sheet because of excessivefilament breakage.

Industrial Applicability

The sheet of PPS filaments of this invention is superior in heatresistance, chemical resistance, flame retardance, electrical insulatingproperties, and mechanical strength. When it comes to heat resistanceover a long period of time, the sheet of this invention is comparable toClass F films.

The sheet of this invention is not attacked by any solvent at lower than200° C. Because of these characteristics, the sheet will find use asindustrial filters, gaskets, packings, firemen's wear, reinforcementsubstrates, heat insulating materials, etc. if made bulky and flexible;and as electrical insulating materials, speaker cones, circuit boards,battery separtors, etc. if made compact.

What is claimed is:
 1. A sheet of polyphenylene sulifide filaments whichcomprises randomly dispersed and accumulated polyphenylene sulfidefilaments, each having a fineness of 0.1 to 15 denier, a dry heatshrinkage of about 5-50% at 140° C. and a strength greater than 1.5 g/d,wherein said filaments are made of a linear polymer having a degree ofcrosslinking and branching as defined by the non-Newtonian constant n of0.9<n<2.0 as defined by the formula ##EQU2## wherein r is shear rate, Tis shear force and μ is viscosity.
 2. A sheet of polyphenylene sulfidefilaments as claimed in claim 1, wherein said p-phenylene sulfide iscopolymerized with a comonomer selected from the group consisting ofm-dichlorobenzene, 1,2,4-trichlorobenzene, compounds having adiphenylether group, compounds having a diphenyl sulphone groups andcompounds having a naphthalene nucleus.
 3. A sheet of polyphenylenesulfide filaments as claimed in claim 1, wherein said PPS filaments havea shrinkage of 5 to 40% at 140° C.
 4. A sheet of polyphenylene sulfidefilaments as claimed in claim 1, wherein said polyphenylene sulfidefilaments are bonded with at least one thermosetting resin selected fromthe group consisting of polyimide, polyamide-imide, aromatic polyamide,polybenzimidazole, and polyarylene sulfide.
 5. A sheet of polyphenylenesulfide filaments as claimed in claim 4, wherein the quantity of saidthermosetting resin is 5 to 90 wt% based on the quantity of thefilaments.
 6. A sheet of polyphenylene sulfide filaments as claimed inclaim 1, wherein said filaments are felted by interlocking.
 7. A web ofpolyphenylene sulfide filaments to be bonded or interlocked comprisingrandomly dispersed and accumulated polyphenylene sulfide filaments,wherein said filaments are 0.5 to 5 denier, said web has an areashrinkage of 5 to 80% and, said filaments have a dry heat shrinkage ofabout 5-40% at 140° C. and a strength greater than 1.5 g/d, saidfilaments are made of a linear polymer having a degree of cross-linkingand branching as defined by the non-Newtonian constant n of 0.9<n<2.0 asdefined by the formula ##EQU3## wherein r is shear rate, T is shearforce and μ is viscosity.
 8. A web of polyphenylene sulfide filaments asclaimed in claim 7, wherein said web weighs 10 to 600 g/m².
 9. A web ofpolyphenylene sulfide filaments as claimed in claim 7, wherein said webhas an area shrinkage of 10 to 60%.
 10. A web of polyphenylene sulfidefilaments as claimed in claim 7, wherein said web has an area shrinkageof 15 to 40%.
 11. A web of polyphenylene sulfide filaments as claimed inclaim 7, wherein said web weighs 20 to 300 g/m².
 12. A process forproducing a sheet of polyphenylene sulfide filaments which comprisesextruding a polyphenylene sulfide polymer from a plurality of smallholes at a temperature 20° to 85° C. higher than the melting point ofthe polyphenylene sulfide polymer, drawing apart the extrudate from thesmall holes at a rate greater than 1300 m/min by a high-velocity airstream, simultaneously causing the resulting filaments to be opened byelectrostatic charge, collecting the opened filaments on a plane, andbonding or interlocking the collected filaments.
 13. A process forproducing a sheet of polyphenylene sulfide filaments as claimed in claim12, wherein said PPS polymer is composed of more than 90 mol% ofp-phenylene sulfide units and has "n" which is represented by 0.9<n<3.0(where n is as defined in the specification).
 14. A process forproducing a sheet of polyphenylene sulfide filaments as claimed in claim13, wherein said polyphenylene sulfide polymer has "n" which isrepresented by 0.9<n<2.0.
 15. A process for producing a sheet ofpolyphenylene sulfide filaments as claimed in claim 12, wherein saidpolyphenylene sulfide polymer has a melt viscosity of 300 to 100,000poise at a shear rate of 200 sec⁻¹ at 300° C.
 16. A process forproducing a sheet of polyphenylene sulfide filaments as claimed in claim12, wherein the extrudate is drawn apart from the small holes at a rategreater than 1300 m/min by a high-velocity air stream.
 17. A process forproducing a sheet of polyphenylene sulfide filaments as claimed in claim12, wherein the extrusion temperature is 30° to 60° C. higher than themelting point of the polyphenylene sulfide polymer.
 18. A process forproducing a sheet of polyphenylene sulfide filaments as claimed in claim12, wherein the interlocking is performed by needle punching.
 19. Aprocess for producing a sheet of polyphenylene sulfide filaments asclaimed in claim 12, wherein the interlocking is performed by water jet.20. A process for producing a sheet of polyphenylene sulfide filamentsas claimed in claim 12, wherein a heat bonding process and a compactingprocess using a hot calendering machine are added.